The invention relates to a magnetic alternative to mechanical snap-action switches which have been used for many years, for example, to monitor the actual position of actuators and fluid controls.
Mechanical snap-action switches, sometimes called microswitches and/or limit switches, present many problems and limitations, particularly miniaturized versions of such switches when needed to monitor the positional condition of a small valve member, or when needed for operation in a corrosive atmosphere. Such mechanical switches are often less reliable than the devices they monitor, as when space launches and aircraft departures have been delayed and sometimes scrubbed because of faulty signals from these mechanical switches during preflight check-out procedures. If a valve effluent is any other than dry inert gas (which it often is), the valve-position-monitoring switch must be sealed from effluent contact, to avoid corrosion of the switch mechanism or an explosion (or fire) hazard; such conditions introduce the cost and complexity of a diaphragm or bellows seal.
Miniaturized mechanical switches are not suitable for use with miniature actuators and valves weighing less than 100 grams because of their size and/or weight, and often because they require relatively great actuating force. Moreover, the stroke requirements of such micro-switches, namely, due to pre-travel, travel and post-travel, are often greater than the position change being monitored.
Even in application to valves which are not miniaturized, miniature mechanical switches usually give only crude indications of actual valve-member position, for example, "open" and "not open", which is to be compared with the desired information "open" and "closed".
Some manufacturers have used magnetic-reed switches in an effort to overcome problems of miniaturized mechanical switches. The magnetic-reed switch can solve the effluent-sealing and safety problems, because each is hermetically encapsulated in a glass envelope. But other problems remain in application of reed switches to miniature valves and actuators; these other problems include poor reliability, high cost, and excessive size.
More recently, electronics have been employed to sense valve and/or actuator position, by non-contacting, non-mechanical means. Proximeters and other solid-state position sensors are now on the market, and U.S. Pat. No. 3,789,876 is an example of a variable-inductance position-sensing technique, specific to a bi-latch style of latching valve.